Determining fluid properties from pressure, volume and temperature

Abstract

A method of determining an amount of dissolved gas in a fluid sample is disclosed. The method includes hydraulically confining and expanding the sample while measuring pressure and volume. A first volume and pressure are determined at which the rate of change in pressure with respect to volume deviates from a linear relationship. A second volume and pressure at which continued expansion of the fluid causes substantially no change in pressure is determined. The first pressure and volume are extrapolated by the linear relationship to intersect an extrapolation from the second volume and pressure at no change in pressure, thereby determining a bubble point comprising a bubble point pressure and a bubble point volume. A third pressure corresponding to measured pressure of the fluid sample at the bubble point volume is determined. An extrapolated sample volume is determined at the third pressure by extrapolating the linear relationship from the bubble point to a sample volume along the linear relationship corresponding to the third pressure. A volume of gas dissolved in the fluid sample is determined by linearly scaling a difference between the bubble point volume and the extrapolated sample volume with respect to a difference between the second volume and the bubble point volume. In a preferred embodiment of the invention, the sample is withdrawn into a sample chamber in an electric wireline formation test tool having a means for measuring pressure and volume of the sample chamber.